Miniaturization and increased processing power has recently allowed great increases in the portability of electronics. Complex devices have been reduced to pocket size. Consumers are able to carry and use wherever they desire such devices as cellular telephones, music players, game players, still and motion digital cameras, and GPS locators. Still, the size and form factor of the devices is often limited by the optical display because most devices currently use an inflexible glass optical display. Such displays are bulky, fragile, and difficult to replace when damaged.
To obtain even greater portability and avoid the drawbacks of glass optical displays, rollable displays have been developed. FIG. 1 is a front view of an exemplary use of a rollable display as disclosed in International Publication Nos. WO 2004/001704 A1 and WO 2004/031927 A2 to Bas J. E. van Rens, both entitled “Apparatus with Display” and both incorporated herein by reference in their entirety. In this embodiment, a rollable display 3 is unrolled from a housing 4,5 into the usable position illustrated. The mobile telephone 1 provides the input to the rollable display 3. In an alternative embodiment, the electronics providing the input to the rollable display, such as the mobile telephone or other electronics, is enclosed in one of the housings.
FIG. 2 is a cross section view of a rollable display device. The rollable display device 10 includes an inner housing 12, such as an inner cylinder, housing the electronics 14, and an outer housing 16, such as an outer cylinder, disposed about the inner housing 12. The wall of the inner housing 12 forms an inner slot 13 and the wall of the outer housing 16 forms an outer slot 17. A rollable display 18 includes a connection portion 20, a bend portion 22, a rolled portion 24, and an external portion 26. The rollable display 18 is connected to the electronics 14 by the connection portion 20, passes through the inner slot 13 at the bend portion 22, wraps around the inner housing 12, and exits the outer housing 16 at the external portion 26. In operation, the rollable display 18 is deployed for use by extending the rolled portion 24 through the outer slot 17. The rolled portion 24 is extended and retracted manually or by motor power.
One problem with rollable display devices is that the repetitive strain of rolling and unrolling the rollable display for use damages the rollable display. The rolled portion 24 typically makes a gradual, large-radius curve around the inner housing 12, so the stress and resulting strain is limited below a design threshold, preventing damage. A radius greater than 7 millimeters for a 100 μeter thick rollable display typically holds strains in the electrically active region of the rollable display below a 1 percent limit. However, the sharp bending of the display 18 at the bend portion 22 exceeds the design threshold and damages the rollable display 18 over time from repetitive strain. Examples of display failures include broken circuits, layer delamination, cracking, and buckling. Such failures may prevent the rollable display from rolling and unrolling, may reduce the image quality of the rollable display, or may prevent the rollable display from functioning. Excessive strain in the bend portion 22 may also interfere with the connection of the rollable display 18 to the electronics 14 at the connection portion 20.
FIG. 3 is a cross section view of a portion of a rollable display. In the example shown, the rollable display 30 includes a first layer 32, a second layer 34, and a third layer 36. Typically, the layers are laminated together to form the rollable display 30. The first layer 32 is typically a substrate for supporting the much thinner second layer 34, which is typically a fragile electronics layer. The third layer 36 is typically an electronic ink layer responsive to electrical charge from the second layer 34 to display text or graphical information to the user. The rollable display 30 has a neutral line 38, which is free of stress when the rollable display 30 is bent, such as when bending the rolled portion around the inner housing or the bend portion through the inner slot. The location of the neutral line 38 depends on the thickness and modulus of elasticity of each layer making up the rollable display 30. When bent, the rollable display 30 is in tension on one side of the neutral line 38 and in compression on the other side. Repeated cycling of the rollable display 30 to a radius smaller than the minimum design radius can result in failure of the rollable display 30.